1. Field of the Invention
The present invention relates to both method and system for recovering polyethylene from scrap material containing polyethylene.
2. Description of the Related Art
With the development of the the petrochemical industry, the production of various synthetic resins such as polyethylene, polypropyrene and polyvinylchloride, which originate from petroleum, has increased.
In particular, use of polyethylene among those synthetic resins has become widespread as a material for various household goods, protective cases for goods, parts thereof, other synthetic fibers, non-woven fabric or wrapping material.
Using products made of polyethylene enables humans to enjoy convenient daily life. However, industrial wastes originating from synthetic resins during the production of goods, or wastes goods made of synthetic resin raises a problem of environmental pollution. That is, air or soil pollution may become serious by incinerating or burying the waste of synthetic resin. Also, the continuous disposal of synthetic resin made from a limited amount of petroleum appears to be draining the petroleum supply, with a possible result being an end to growth in the synthetic resin industry.
Accordingly, a method for recovering synthetic resin from scrap material has been suggested so as to solve the above problems.
For example, Korean Patent Publication No. 89-4486 discloses methods for separating and recovering polyethylene from scrap thin metal plates coated with polyethylene resin. In the above disclosure, the thin metal plate covered with polyethylene resin is cut into an appropriate size for processing, and heated to a temperature higher than the melting point of polyethylene, to melt the polyethylene resin. As a result, the polyethylene resin is separated from the thin metal plate. However, in the polyethylene separation method, the entire system must be maintained at a high temperature to completely separate the molten polyethylene having a high viscosity from the thin metal plate, so that energy consumption increases. Also, the method is not suitable for scrap thermosetting resin or compound synthetic resin having synthetic resins whose difference in melting points is not great.
In addition, Korean Patent Publication No. 91-7451 and Laid-open Publication No. 95-8071 disclose methods for separating and recovering polyethylene resin using polyethylene-soluble solvent from scrap electric cables coated with polyethylene resin. Also, Japanese Patent Laid-open Publication No. 6-287350 discloses methods for separating and recovering polyolefin using polyolefin-soluble solvent from scrap materials containing polyolefin.
However, in the case of separating polyethylene from scrap materials only using a solvent as in the above disclosures, the polyethylene solution must be heated to a temperature higher than the boiling point of the polyethylene-soluble solvent, so as to separate the solvent. That is, if the polyethylene solution having only polyolefin-soluble solvent is cooled, polyethylene swells and is gelated in the solvent, thus it is difficult to separate polyethylene by filtration. However, evaporating the solvent requires much heat energy. Also, as the solvent evaporates, the concentration of polyethylene in the polyethylene solution relatively increases, so that the evaporation of solvent is hindered. Accordingly, a considerable amount of solvent remains in the recovered polyethylene. Thus, the quality of recovered polyethylene is lowered. Also, due to the incomplete recovery of the solvent, the production cost increases and the entire process becomes longer.
Japanese Patent Laid-open Publication No. 9-208738 discloses methods for depositing and recovering polyolefin from scrap materials containing polyolefin, in which polyolefin is dissolved in a polyolefin-soluble solvent, an insoluble component is removed from the mixture, and then a depositing agent or a poor solvent is added to polyolefin solution. However, extra processes for separating the polyolefin-soluble solvent and the depositing agent or poor solvent are required, so that energy consumption and the production cost increases in addition to the entire process being longer.